/^.$/us is transpiled incorrectly

[thorn0]

Current output:

const transpiled = /^(?:[\0-\uD7FF\uE000-\uFFFF]|[\uD800-\uDBFF][\uDC00-\uDFFF]|[\uD800-\uDBFF](?![\uDC00-\uDFFF])|(?:[^\uD800-\uDBFF]|^)[\uDC00-\uDFFF])$/

Actual result:

transpiled.test('\u0001\udc00'); // true

Expected result:

/^.$/us.test('\u0001\udc00'); // false

[jridgewell]

It's last branch (?:[^\uD800-\uDBFF]|^)[\uDC00-\uDFFF] which is causing the error. (?:[^\uD800-\uDBFF]|^) is eating the \u0001 and [\uDC00-\uDFFF] is matching the \udc00. See #17 for context why this happens, and mathiasbynens/regenerate#28. Unfortunately, it's not possible to implement correctly without lookbehinds.

[thorn0]

Okay, I just wanted to report a bug. If it's a known thing, feel free to close the issue.

[jridgewell]

/cc @mathiasbynens

[mathiasbynens]

This is the fourth bullet point on https://github.com/mathiasbynens/regexpu#known-limitations:

4. regexpu doesn’t match lone low surrogates accurately. Unfortunately that is impossible to implement due to the lack of lookbehind support in JavaScript regular expressions.

JSC/Safari still doesn't support lookbehinds, so this phrasing is still accurate even for projects that only need to consider modern JS environments. :(